Combustion, an example of chemical reaction
After the prehistoric human learned to master fire, it was realized that it modified some materials around it.
It was possible, for example, to realize that some materials burn and others do not. It has also been found that certain foods, if baked, would taste more pleasant.
Objects of wet clay, when dried on fire, became rigid and impermeable, as opposed to simply drying on the sun; therefore they were more useful. The technique for producing ceramic objects, still used today in the production of bricks, tiles, vases, pots, etc. was discovered.
By heating certain minerals, ancient peoples discovered that it was possible to obtain copper metal from copper ore and, centuries later, iron from iron ore.
To realize that a transformation has taken place, scientists choose a certain system and observe it over time. System is a portion of matter that has been chosen to be studied, observed. A system may consist of a pure substance or a mixture of substances. It can be as small as a droplet of liquid examined under a microscope or as large as the planet's atmosphere.
To verify the occurrence of transformation in a system, it must be described at different times. When we consider only two observations of a system made at different times, we say that the first refers to the initial state of the system and the second to the final state of the system.